01 - Preheat the oven to 350°F (180°C). Lightly grease a medium baking dish.
02 - Steam or blanch the broccoli florets and sliced carrots for 3 to 4 minutes until just tender. Drain well and set aside.
03 - In a medium saucepan over medium heat, melt 2 tablespoons butter. Add flour and whisk continuously for 1 minute until foaming but not browned.
04 - Gradually pour in the milk, whisking until the mixture becomes smooth and thickened. Remove the pan from heat, then stir in shredded cheddar, garlic powder, onion powder, salt, and black pepper until the cheese is fully melted and the sauce is creamy.
05 - Gently fold the blanched broccoli and carrots into the cheese sauce, ensuring even coating. Transfer the mixture to the greased baking dish.
06 - In a small bowl, mix panko breadcrumbs with melted butter and, if using, grated Parmesan cheese. Sprinkle the topping evenly over the vegetables in the baking dish.
07 - Bake for 25 to 30 minutes, or until the topping is golden brown and the casserole is bubbling. Allow to rest for 5 minutes before serving.